Molecular gas in galaxies of Hickson compact groups

Abstract

We have observed 70 galaxies belonging to 45 Hickson compact groups in the ¹² CO(1 → 0) and ¹² CO(2 → 1) lines, in order to determine their molecular content. We detected 57 galaxies, corresponding to a detection rate of 81%. We compare the gas content relative to blue and L_FIR luminosities of galaxies in compact groups with respect to other samples in the literature, including various environments and morphological types. We find that there is some hint of enhanced NH₂ /L_B and M_dust /L_B ratios in the galaxies from compact group with respect to our control sample, especially for the most compact groups, suggesting that tidal interactions can drive the gas component inwards, by removing its angular momentum, and concentrating it in the dense central regions, where it is easily detected. The molecular gas content in compact group galaxies is similar to that in pairs and starburst samples. However, the total L_FIR luminosity of HCGs is quite similar to that of the control sample, and therefore the star formation efficiency appears lower than in the control galaxies. However this assumes that the FIR spatial distributions are similar in both samples which is not the case at radio frequencies. Higher spatial resolution FIR data are needed to make a valid comparison. Given their short dynamical friction time-scale, it is possible that some of these systems are in the final stage before merging, leading to ultra-luminous starburst phases. We also find for all galaxy samples that the H₂ content (derived from CO luminosity and normalised to blue luminosity) is strongly correlated to the L_FIR luminosity, while the total gas content ( H₂+HI ) is not.